A pacemaker is implantable cardiac stimulation device for implant within a patient that analyzes an intracardiac electrogram (IEGM) to detect various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An implantable cardioverter-defibrillator (ICD) additionally detects atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation.
State of the art pacemakers and ICDs are also capable of detecting and tracking a wide range of physiological parameters relevant to the health of the patient. One parameter of particular interest is arterial blood pressure, which is relevant to a variety of medical conditions such as hypotension. However, it is difficult to reliably measure arterial blood pressure with a pacemaker or ICD. Heretofore, some attempts have been made to develop implantable blood pressure sensors employing piezoelectric devices, or the like, but with limited success. Such sensors can clog or otherwise degrade over time. One example of a technique for sensing blood pressure via an implantable medical device is described in U.S. Patent Application 2003/0158584 of Cates et al., entitled “Chronically-Implanted Device for Sensing and Therapy.” In view of the problems arising with implantable blood pressure sensors, arterial blood pressure is usually detecting using an external blood pressure sensor, such as conventional blood pressure cuff-based device. Although external sensors can provide accurate measurement of arterial blood pressure, such measurements are, at best, infrequent. In addition, with blood pressure detector externally, the pacemaker or ICD cannot utilize the detected blood pressure value to automatically adjust therapy, unless the value is also automatically transmitted to the device. Conventional external blood pressure sensors do not provide that capability.
It would be far preferable to provide a technique for continuously detecting arterial blood pressure using a pacemaker or ICD. Ideally, such a technique would utilize an external blood pressure sensor only for occasional recalibration. Moreover, such a technique would preferably not require any sensors to be implanted within patient arteries. In particular, it would be highly desirable to provide a technique for detecting arterial blood pressure using a pacemaker or ICD that can exploit otherwise conventional pacing leads. The present invention is directed to providing just such a technique.
Once arterial blood pressure has been detected, it can be used in combination with other parameters, such as heart rate, to detect stroke volume and cardiac output within the patient. Cardiac output is a particularly important parameter to track within patients with congestive heart failure (CHF) or within patients who are at risk of CHF. Hence, additional aspects of the invention are directed to detecting stroke volume, cardiac output and other physiological parameters in conjunction with the detection of arterial blood pressure.